Flares from spiral waves by lensing and time-delay amplification?
V. Karas, M. Dovciak, A. Eckart, L. Meyer
备注：
Proceedings of the Workshop on the Black Holes and Neutron Stars, eds. S. Hledik and Z. Stuchlik, 19-21 September 2007 (Silesian University, Opava), in preparation
发表信息：
Proceedings of the Workshop on the Black Holes and Neutron Stars, eds. S. Hledik and Z. Stuchlik, 19-21 September 2007 (Silesian University, Opava), pp. 99-108
Episodically accreting black holes are thought to produce flares when a chunk of particles is accelerated to high velocity near the black hole horizon. This also seems to be the case of Sagittarius A* in the Galactic Center, where the broad-band radiation is produced, likely via the synchrotron self-Compton mechanism. It has been proposed that strong-field gravitational lensing magnifies the flares. The effect of lensing is generally weak and requires a fine-tuned geometrical arrangement, which occurs with only a low probability. However, there are several aspects that make Sagittarius A* a promising target to reveal strong gravity effects. Unlike type II (obscured) active galaxies, chances are that a flare is detected at high inclination, which would be favourable for lensing. Time delays can then significantly influence the observed flare duration and the form of light-curve profiles. Here we discuss an idea that the impact of lensing amplification should be considerably enhanced when the shape of the flaring clump is appropriately elongated in the form of a spiral wave or a narrow filament, rather than a simple (circular) spot which we employed previously within the phenomenological `orbiting spot model'. By parameterizing the emission region in terms of the spiral shape and contrast, we are able to extend the spot model to more complicated sources. In the case of spirals, we notice a possibility that more photons reach a distant observer at the same moment because of interplay between lensing and light-travel time. The effect is not symmetrical with respect to leading versus trailing spirals, so in principle the source geometry can be constrained. In spite of this, the spot model seems to provide entirely adequate framework to study the currently available data.
A universal probability distribution function for weak-lensing amplification
我们提出了一个近似形式的弱透镜放大率分布的标准蜡烛，适用于所有的宇宙模型，与任意的物质分布，在所有的红移。我们的结果是基于一个通用的概率分布函数（UPDF），$P(η)$，减少收敛，$η$。对于一个给定的宇宙学模型，放大概率分布$P(μ)$在红移$z$处与UPDF by $P(μ)=P(η)/2|κ_{min}|$相关，其中$η=1+(μ-1)/(2|κ_{min}|)$和$κ_{min}$（最小收敛）可以直接从宇宙学参数（$Ω_m$和$Ω_Λ$）计算。我们表明，UPDF可以很好地近似由三个参数的拉伸高斯分布，其中的三个参数的值仅取决于$ξ_η$，方差的$η$。简而言之，所有可能的弱透镜概率分布都可以很好地近似为一个单参数族。我们建立了这个族，归一化为Wambsganss等人（1997）的$Λ$ CDM模型的数值射线射击结果。每一个可供选择的宇宙学模型都可以用一个函数$ξ_η(z)$来描述。我们发现，这种方法给出了$P(μ)$在良好的协议与数值射线追踪和三维剪切矩阵计算，并提供了三个代表性的模型（SCDM，$Λ$ CDM，和OCDM）的数值拟合。我们的结果提供了一个简单，准确，有效的方法来计算弱透镜放大率分布的标准蜡烛，并应在未来的高红移超新星数据的分析有用。
